Degeneration of the intervertebral disc within the spine is generally believed to be a common cause of debilitating lower back and neck pain. An intervertebral disc primarily serves as a mechanical cushion between the vertebral bones, permitting controlled motions within vertebral segments of the axial skeleton. The normal disc is a unique, structure, comprised of three component tissues: the nucleus pulposus (“NP”), the annulus fibrosus (“AF”), and the cartilaginous end plates of the two opposing vertebral bodies. The configuration of the healthy disc is such that the NP, a soft gelatinous material, is situated in the center of the disc while the AF, a tough, laminated ring of crisscrossing layers, surrounds and contains the NP. The disc is connected to the superior and inferior vertebrae through hyaline cartilage-based vertebral end plates that are approximately 1 mm thick and serve as a semipermeable membrane.
The AF is a tough annular shaped fibrocartilage tissue which consists mainly of Type 1 collagen fibers which are organized into many crisscrossed layers forming a tough, outer fibrous ring that binds together adjacent vertebrae. Approximately 60-70% of the mass of the AF is water. This fibrous portion, which is shaped much like a laminated automobile tire, is generally about 10 to 15 millimeters in height and about 15 to 20 millimeters in thickness. The AF consists of overlapping multiple plies at roughly a 30-degree angle with respect to the radial direction that are sequentially oriented to alternate in direction. The fibers of the AF are connected to the vertebral end plates as well as being directly bound to the superior and inferior vertebral bodies. This configuration particularly resists torsion, as about half of the angulated fibers will tighten when the vertebrae rotate in either direction, relative to each other. The laminated plies are less firmly attached to each other. This configuration ensures significant resistance against radial stress and inner over-pressure, while allowing significant deformation during twisting and bending.
The AF disc contains a complex flexible and hydrophilic core, the nucleus pulposus (NP). The NP consists of a gel-like composite made of proteoglycans (PGs) and Type II collagen. The NP resides in the center of the AF and the transition between these two tissues is quite distinct at birth but becomes more gradual with increasing age. The high PGs content, as much as 65% for young individuals allows it to maintain a water content of more than 90% of its total mass. PGs possess glycosaminoglycan chains with ionic carbonyl and sulfate groups that have the ability to attract and retain water molecules. The NP absorbs water rapidly when load is applied to the spine (sitting up, standing, hip rotation, walking, etc.) serving as a pump that takes up and expels water depending on the pressure within the disc. In this manner, the degree to which the disc is loaded with external forces determines the amount of water in the NP. For example, if the disc is under increased compression, the pressure within the disc increases and water is forced out of the NP. When the load on the disc decreases, the pressure within the disc lessens and water is allowed to flow back in. This phenomenon is an effective mechanism for providing the exchange of waste and nutrients through the vertebral end plates. This is particularly critical for cells that reside within the disc since the disc is a largely avascular structure, having no direct blood supply. A healthy NP is largely a gel-like substance having a high water content, and similar to the air in a tire, serves to keep the annulus tight in tension yet flexible enough to allow some degree of motion.
The complex structure of the intervertebral disc performs the important role of absorbing mechanical loads while allowing for constrained flexibility of the spine. A healthy NP is critical to the disc function and the normal load transfer mechanism that occurs within the spine. In particular, the swelling pressure generated by the NP transmits external forces that act on the disc to the AF. For example, an axial load acting on the disc causes the intradiscal pressure within the NP to increase thereby creating tension on the surrounding ring shaped AF, pushing it outward and preventing it from bulging inward. When the fibers of the AF are stretched, they are strengthened to better resist the vertical loading on the disc.
With increased aging, degenerative changes naturally occur within the disc. The term, degenerative disc disease (DDD), refers to degradation of normal disc architecture into a pathological state. It has been previously reported that by age 50, nearly all intervertebral discs have undergone some degree of degeneration. The onset of DDD is believed to occur as the NP begins to lose its ability to retain water. This is due to a decrease in the PGs content within the NP of the disc as well as changed in the PGs chemical composition. More specifically, the PGs composition is modified as the ratio of keratin sulfate to chondroitin sulfate increases. The changes result in the PGs composing approximately 65% of the dry weight of the NP in young individuals to less than 30% with aging. This impacts the water binding capacity of the NP as its water content may decline from about 90% at birth to about 70% or less in old age. There is also an associated decrease in the number of resident cells within the NP tissue. With the decreased water content and cellularity, the NP loses volume and becomes less gel-like and more fibrous in nature and the border between the NP and the AF becomes much less distinct. This transformation of the NP within the disc is similar to the air leaking from a tire.
As the DDD evolves, the load transfer mechanism of the disc is significantly modified. With these pathologic changes, the NP can no longer effectively transfer loads and provide sufficient pressurization to keep the AF in tension. When not properly tensioned, the layers of the AF do not have the same ability to resist compressive loads and experience atypical stresses. Without a healthy NP to resist the AF from bulging inward, this abnormal stretching of the AF causes this tissue structure to weaken by making the successive plies buckle and separate from each other. This causes the AF to become more susceptible to radial fissures or cracks under loading. Over time, the disc also loses stability and height bringing the spinal facet joints in close contact with each other.
Following a full-thickness tear in the AF, the NP is no longer prevented from escaping from the disc under loading. NP material then moves through the crack in the annulus and reaches the outside of the disc where it may cause inflammation and come into contact with a nerve root. This phenomenon is often referred to as “herniated” disc with the nerve impingement typically resulting in debilitating back or leg pain, loss of muscle control or even paralysis. The most common resulting symptoms are pain radiating along a compressed nerve and low back pain, either of which can be crippling for the patient. The significance of this problem is increased by the low average age of diagnosis with over 80% of patients in the United States being under 59.
While conservative care is frequently the first treatment option, surgical solutions are often necessary to alleviate pain and discomfort. When conservative approaches are not successful, the most common surgical options are currently discectomy and spinal fusion. While both of these options are reasonably successful at acutely decreasing pain, neither one restores proper biomechanics to the spine, which may lead to further degeneration at the operated disc or discs at the adjacent levels in the spine.
Since 1934, discectomy has been utilized as a common surgical procedure for treating intervertebral disc herniation. This procedure is performed with the AF still relatively intact and involves removal of disc materials impinging on the nerve roots or spinal cord external to the disc, generally posteriorly. Depending on the surgeon's preference, varying amounts of NP are then removed from within the disc space either through the herniation site or through an incision in the AF. This removal of extra NP further diminishes the volume of the NP but is commonly done to minimize the risk of recurrent herniation.
The most significant drawbacks of discectomy are recurrence of herniation, recurrence of radicular symptoms, continuing loss of disc height and increasing low back pain. Re-herniation can occur in a significant number of cases. The site for re-herniation is most commonly the same level and side as the previous herniation and can occur through the same weakened site in the AF. Persistence or recurrence of radicular symptoms happens in many patients and when not related to re-herniation, tends to be linked to stenosis of the neural foramina caused by a loss in height of the operated disc. All of these failings are most directly related to the loss of NP material and AF competence that results from herniation and surgery.
Loss of NP material via discectomy further deflates the disc, causing a decrease in disc height. Loss of disc height increases loading on the facet joints. This can result in deterioration of facet cartilage and ultimately osteoarthritis and pain in this joint. As the joint space decreases the neural foramina formed by the inferior and superior vertebral pedicles also close down. This leads to foraminal stenosis, pinching of the traversing nerve root, and recurring radicular pain. Loss of NP also increases loading on the remaining AF, a partially ennervated structure that can produce pain. Finally, loss of NP results in greater bulging of the AF under load. This can result in renewed impingement by the AF on nerve structures posterior to the disc.
Persisting tears in the AF that result either from herniation or surgical incision also contribute to poor results from discectomy. The AF has limited healing capacity with the greatest healing occurring in its outer borders. Healing takes the form of a thin fibrous film that does not approach the strength of the uninjured disc. Surgical incision in the AF has been shown to produce immediate and long lasting decreases in stiffness of the AF particularly against torsional loads. This may over-stress the facets and contribute to their deterioration. Further, in as many as 30% of cases, the AF never closes. In these cases, not only is re-herniation a risk but also leakage of fluids or solids from within the NP into the epidural space can occur. This has been shown to cause localized pain, irritation of spinal nerve roots, decreases in nerve conduction velocity, and may contribute to the formation of post-surgical scar tissue in the epidural space.
Spinal fusion is a common surgical treatment option for patients that have persistent back pain and whose annulus is severely compromised. This procedure involves removing a majority of the disc and causing bone to grow between the two adjacent vertebrae. If successful, this results in the two vertebrae being “fused” together This treatment generally reduces back pain but limits the mobility of the spine. It is suspected that this abnormal biomechanical loading may lead to DDD and repeat surgeries at the adjacent levels.
All present surgical interventions, whether laminectomy or fusion of adjacent vertebrae, lower the functionality of the spine in some way. For that reason it is desirable to try to develop a prosthetic for the spinal disc or its parts. This is, however, extremely difficult. The spine is a very complex part of the body and its proper function is dependent on proper coordination of the function of all the parts, including the spinal discs. The spinal disc needs to withstand complex stresses, including various angles of bending, pressure, shear, and twisting. The spinal disc must also function as a shock and vibration absorber. And finally, a spinal disc must allow the transport of the nutrients and metabolic products needed for its health and survival.
There have been a number of attempts to try to correct or repair the problems connected to defective spinal discs. The first prostheses embodied a wide variety of ideas primarily using mechanical devices such as ball bearings, springs, metal spikes and other perceived aids. These prosthetic discs were designed to replace the entire intervertebral disc space, and were large and rigid. Beyond the questionable efficacy of those devices were the inherent difficulties encountered during implantation.
A new procedure has been developed which is a mechanical, motion-preserving device replacing the natural interbody joint. The mechanical disc is based on the highly successful hip or knee prostheses; these have metal on plastic or metal on metal rotating or sliding elements. These mechanical discs are in the early stages of clinical evaluation and are relatively unproven. Concerns exist based on the metal/plastic interface which would result in fine plastic particles being created in the delicate disc space adjacent to the spinal chord. These plastic debris particles have caused serious complications in the knee and hip applications.
The construction of a fully functional prosthesis is extremely difficult and most prosthetic devices suggested to date are strictly mechanical, and they mimic only some functions of the disc. A prosthetic with a simulation of the disc function is shown in U.S. Pat. No. 4,911,718 issued Mar. 27, 1990 describing a composite construction of the prosthetic of the disc using a biocompatible elastomer, reinforced by fibers which mimic the function of collagen fibers in a natural spinal disc. One disadvantage of this solution, which is common to all full spinal disc replacements, remains a complicated surgical procedure, which translates into a high cost, and a high risk to the patient.
Another surgical approach to restore natural biomechanics in the spine for patients with DDD is augmentation or replacement of the disc nucleus. Here, rather than replacing the entire disc, only the central core of the disc is modified. This preserves the surrounding structures of the disc including the annulus as well as the cartilaginous end plates. The procedure is less complicated and less invasive than TDR therapies. However, this approach does require the AF to be sufficiently intact to contain the NP implant.
The first disc nucleus replacements implant into humans were stainless steel balls developed by Fernstrom in 1966. These solid implants did not restore proper biomechanics in discs due to their stiffness. In addition, some implants migrated from the disc space or subsided into the vertebral end plates.
U.S. Pat. No. 5,047,055 issued Sep. 10, 1991 describes a hydrogel prosthesis of the nucleus, whose shape and size corresponds to the removed disc nucleus when the prosthesis is fully swollen. The prosthetic is prepared in a partially dehydrated state when the dimensions are smaller and the device can be inserted through a smaller opening. After implantation, the prosthesis will grow to its full size by absorbing bodily fluids. It is necessary to note, however, that the dehydration prior to implantation and rehydration after implantation are isotropic, i.e. all dimensions change at the same rate. During implantation the implant will try to expand equally in all directions, but it will expand most in the direction of the least resistance. Therefore it will expand the least in the axial direction, where expansion is most needed (so that the separation of the vertebrae is the highest), and it will expand the most in the radial direction, where the expansion is least desirable; especially in places where the AF is weakened or even missing.
The use of expandable materials in a prosthetic element is also disclosed in U.S. Pat. No. 5,545,222 issued Aug. 13, 1996. Such materials which expand when they come in contact with water or other fluids include PEEK (polyether-etherketone), a desiccated biodegradable material, or a desiccated allograft. As an example, a tendon can be compressed in a desiccated state, and as it imbibes water it expands and creates a firmer lock or tighter fit in the host site.
A shaped, swollen demineralized bone and its use in bone repair is disclosed in U.S. Pat. No. 5,298,254 issued Mar. 29, 1994. In general, cortical allogeneic bone tissue is preferred as the source of bone. Demineralized bone is contacted with a biocompatible swelling agent for a period of time sufficient to cause swelling of the piece.
U.S. Pat. No. 6,620,196 issued Sep. 16, 2003 is directed toward a nucleus pulposus implant having an elastic body and an outer shell which can take a number of forms including a cylinder, rectangular block, spiral and other shapes having a shape memory. The body can be formed from a wide variety of biocompatible polymeric materials.
U.S. Pat. No. 6,652,593 issued Nov. 25, 2003 discloses a demineralized cancellous bone formed into an implant. The implant is capable of being softened and compressed into a small first shape and hardened in the first shape. The compressed shape is hydrated and expands into a second shape having larger dimensions than the original shape. The demineralized cancellous bone may also be used in nucleus replacement.
U.S. Patent Publication No. 2004/0243242 published Dec. 2, 2004 is directed towards an implant constructed of a demineralized fibular ring placed within the medullary canal of another demineralized femoral ring for replacement of an invertebral disc. The disc implant is placed so that the axis of the medullary canal runs parallel to the axis of loading to provide load bearing capacity.
As previously described, in addition to restoring normal biomechanics within the disc, an important feature of a prosthetic nucleus pulposus implant is that the annulus is not entirely removed upon implantation. Normally, however, an opening of some type must be created through the annulus in order for the device to be inserted. Since the creation of this opening traumatizes the annulus, it is highly desirable to minimize its size. Unfortunately, however, most prosthetic nucleus devices that are designed to be implanted through a small annulotomy do not properly fill the nuclear cavity. On the other hand, a relatively rigid prosthesis configured to approximate a shape of the natural nucleus requires an extremely large opening in the annulus in order for the prosthetic device to “pass” into the nucleus cavity.
Degenerated, painfully disabling spinal discs are a major economic and social problem for patients, their families, employers and the public at large. Any significant means to correct these conditions without further destruction or fusion of the disc will serve an important role and be highly beneficial. Other means to replace the function of a degenerated disc have major problems such as complex surgical procedures, unproven efficacy, placing unnecessary and possibly destructive forces on an already damaged annulus, etc. Therefore, a substantial need exists for a prosthetic spinal disc nucleus formed to facilitate implantation through an annulus opening while providing necessary intradiscal support following implant.